Long-term response and recovery to nutrient addition of a partitioned arctic lake

1. To study the bottom‐up linkages in arctic lakes, we treated one side of a partitioned lake with inorganic nitrogen and phosphorus for a 6‐week period each summer for 6 years starting in the summer of 1985. We took a variety of weekly measurements to determine the impact of the nutrient loading on the lake and continued weekly measurements for 2–6 years after the cessation of nutrient loading to observe the recovery of the treated side. The loading rates (2.91 mmol N m^?2 day^?1 and 0.23 mmol P m^?2 day^?1) were five times the calculated loading rates for Toolik Lake, located nearby. 2. In all 6 years of nutrient addition, phytoplankton biomass and productivity were greater in the treated sector than the reference sector. In the first 4 years of nutrient addition there was no flux of phosphorus from the mineral‐rich sediments. This changed in the last 2 years of nutrient addition as phosphorus was released to the lake. 3. The response of the animal community to increased plant production was mixed. One of the four macro‐zooplankton species (Daphnia longiremis) increased in number by about twofold in the first 5 years. However, the copepod Cyclops scutifer showed no response during the treatment phase of the study. The benthic invertebrate response was also mixed. After a 2‐year lag time the snail Lymnaea elodes increased in the treated lake sector but chironomids did not. 4. Ecosystem response to fertilisation was not controlled solely by nutrient addition because phosphorus was not recycled from the sediments until the last 2 years of nutrient addition. Phytoplankton still showed the effects of nutrient addition in the recovery period and the hypolimnion of the treated sector was still anaerobic starting at 6 m in 1996.     

The Feeding of Amoeba proteus on Paramecium aurelia*

SYNOPSIS. Density of prey (Paramecium aurelia) and predator (Amoeba proteus) were varied while volume of inorganic medium was kept constant. Variations in density of prey had little effect on the rates of feeding and reproduction of the amoebae; but with increasing predator density the amoebae captured the paramecia less rapidly and ingested fewer before dividing, altho division size did not change appreciably. Therefore, amoebae of a low density population with a constant food supply carry more nutritional reserves from generation to generation than do those in a denser population.     

RIBONUCLEIC ACID METABOLISM AND CELL EXPANSION IN OAT COLEOPTILE

RNA metabolism in oat coleoptiles was studied using physiologicalresponses to 5-FU and actinomycin D; autoradiographic detectionof RNA and protein synthesis; and estimation of ribosomal concentrationby analytical ultracentrifugation. 5-FU failed to inhibit growthof either intact coleoptiles or isolated coleoptile segmentsbut completely blocked cell division in roots. Actinomycin Dmarkedly inhibited auxin-induced expansion of coleoptile segments.When supplied to isolated segments from coleoptiles of variouslengths the RNA precursors cytidine, adenine and adenosine allshowed weak incorporation into RNA of nuclei and in some cases,to a lesser extent, RNA of cytoplasm. IAA did not affect thisRNA synthesis but it was considerably reduced by actinomycinD. A proportion of the label incorporated from RNA precursorswas not removable with either RNase, PCA or hot TCA but wasextracted by trypsin. The amount of this spurious incorporationincreased with coleoptile age, as did the ability to incorporatelabelled amino acids. The concentration of both free and boundribosomes does not increase in growing coleoptiles and may evendecline. Free ribosomes decline markedly in fully grown coleoptileswhile the proportion of bound ribosomes increases. It is concludedthat young coleoptiles contain a full complement of ribosomesnecessary for subsequent growth but normal growth is dependenton continued production of an actinomycin D-sensitive messenger-typeRNA. No evidence for auxin mediation of RNA synthesis was found. ¹Present address: Laboratory of Cell Biology, Faculty of Science,Osaka City University, Sugimoto-cho, Sumiyoshi-ku, Osaka, Japan.     

Evidence that Enzyme Polymorphisms are not Selectively Neutral

THE discovery of large amounts of electrophoretically-detectable genetic variation within natural populations has aroused considerable interest in the factors maintaining so much polymorphism. It has been proposed that these allozyme polymorphisms reflect the action of random processes^1–6, the polymorphic variation being of little or no selective significance. Alternative hypotheses have suggested that these polymorphisms may be maintained by a balance of selective forces^7–10.